Amplitude modulation adapter system



Dec. 23, 1947. Y M. G. CROSBY n 2,433;093

AMPLITUDE MODULATION ADAPTER SYSTEM Filed April 27, 1943 I .1. E.; UvA/mamma T ATTCRNEY Patented Dec. 23, 1947 AMPLI'EUDE MGDULATION ADAPTER SYSTEM Murray G. Crosby, Riverhead, N. Y., assigner to Radio Corporation of America, a corporation of Delaware ATENT GFFICE Application April 27, 1943, Serial No. 484,705

(Cl. Z50-20) Claims. v1

My present invention relates to a novel method of, and means for, receiving amplitude modulated carrier wave signals with a receiver primarily designed for receiving angle modulated carrier waves, and more specifically to an amplitude modulation adapter for use with a frequency modulation receiver.

The present trend of broadcast reception appears to be in the direction of what has been generically referred to as angle modulated carrier wave reception. The more specific form of such reception is frequency modulation (hereinafter referred to for brevity as FM). The aforementioned generic term, however, includes phase modulated PM for brevity) reception, or even hybrids of FM and PM carrier waves. It seems obvious that if the trend toward FM transmission continues then home broadcast receivers will be of one of two types. That is to say, the home broadcast receiver will either be designed in effect as two receiving systems, one for amplitude modulation (AM) reception, and one for FM reception. On the other hand, the broadcast receiver could be an FM receiving system. In the latter case AM carrier waves would be difficult to receive. Yet there may be many situations where it will be desirable to radiate AM signal energy in the broadcast band of 550 to 1700 kilocycles (kc.) or in any other frequency bands.

Accordingly, it may be stated that it is one of the main objects of my present invention to provide an adapter system which can be employed in connection with a, conventional FM receiver so as to enable AM signal energy, such as is radiated in the present broadcast band of 550 to 1700 kc.,

to be received eiiectively and with a minimum of circuit changes.

It is an important object of this invention to translate amplitude modulated carrier energy into frequency modulated carrier energy by a simple and highly novel method; the method essentially consisting of combining the amplitude modulated wave energy with unmodulated oscillations of constant amplitude and constant frequency, the latter being located within the frequency band width of a frequency modulation receiying system.

Another important object of this invention is to provide a method of receiving amplitude modu lated carrier energy by changing the frequency of received amplitude modulated signal energy, and combining the changed frequency energy with unmodulated oscillations of a frequency Vclose to the changed frequency thereby to producefrequency modulated oscillations of a mean fre- 2 quency equal to the frequency of the unmodulated oscillations.

Other features will best be understood by reference to the following description, taken in connection with the drawing, in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into eifect.

In` the drawing:

Fig. 1 schematically shows a system embodying one form of my invention,

Fig. 2 vectorially illustrates the mode of translating the received AM signal energy into FM signal energy,

Fig. 3 illustrates the amplitude relations be tween the combined unmodulated oscillations and the received frequency-changed AM signal energy,

Figgl shows a modification of the system shown in Fig. 1.

Referring now to the accompanying drawing, wherein like reference characters in the different figures designate similar circuit elements, and particularly to Fig. 1 thereof the numeral l designates a Conventional FM receiver which is schematically represented. rfhe reason for the schematic representation is that those skilled in the art are fully acquainted with the manner of constructing an FM receiver system. It is sufficient to show that the modulation output energy of the receiver I is fed to any desired type of modulation reproducer, as, for example, any reproducer of sound modulation. The numeral 2 designates the usual signal selector input circuit of an FM receiver. This signal selector circuit is simply represented as having an input coil 3 and a variable tuning condenser 4. It should be understood that these numerals 3 and ll do not designate actual physical elements; the elements 3 and .4 are really symbolic insofar as they denote any generalized components which may be used to provide the signal selector circuit. Let it be assumed that the receiver I is of the superheterodyne type, and that the selector circuit 2 is tunable over the present FM broadcast band which covers a range of 40 to 50 megacycles (me). Of course, the selector 2 could be tunable over any other megacycle range utilized for FM reception. A

For the purposes of my invention, as will be more fully discussed at a later point, I provide an auxiliary tuning element 5 such as a condenser of fixed magnitude. The condenser 5 may be connected in parallel with l the input coil 3 by means of a switching device 6. This switching device 6 is arranged for conjoint action with a second switch 1. The latter switch 1 is in circuit with the tuning condenser 4. Mechanical coupling, represented by a dotted line, may be employed between switches 6 and 1 so that upon closure of switch 6 the switch 1 will open, and vice versa. Any well known form of push button mechanism could be employed to actuate the switches 6 and 1 concurrently and in opposite senses. In other words, it will be seen that the input selector circuit 2 may be switched over from its variable adjustment condition to a fixed resonant condition. The condenser may be given a magnitude such that it is capable of tuning coil 3 to some predetermined frequency in the frequency range of selector circuit 2. For example. let it be assumed that condenser 5 tunes the coil 3 to a frequency of 42.4 mc. when switch 6 is closed and switch 1 is open. Let it be further assumed that there is no FM station transmitting at 42.4 mc.

The numeral 8 designates a dipole signal collector which is employed for receiving FM signals in the designated 40-50 mc. band. The dipole collector device is magnetically coupled to the input coil 3. Here, again, for the purposes of my invention the dipole circuit may include a 'switch 9 which is closed when the switch 6 is open and switch 1 is closed. Switch 9 is connected in the leads 8' from dipole 8 to input coil 9 coupled to tuning coil 3. Therefore, when switch 9 is opened no FM signals picked up, or collected, by dipole 8 can be applied to input circuit 2. A second switch I ll is arranged for conjoint mechanical operation with switch 9. A dotted line between the switches 9 and I0 denotes a means for actuating the switches concurrently and in opposite senses. Switch I0 connects an AM adapter system to the FM receiver input circuit. Switch I0 is connected for selectively connecting the upper side of resonant circuit 26 through condenser 21 to the upper side of resonant circuit 2. It will now be seen that for normal FM reception the switch I 0 is open and switch 9 is closed. Concurrently, the switch 1 is closed and switch 6 is open. Of course, the selector circuits of the FM receiver will be given a, pass band which is sufciently wide to transmit the maximum frequency deviation presented to each selector circuit.

For example, under present FM broadcast conditions each selector circuit is designed to transmit a band of some 150 kc. It will be understood that when switch 6 is closed and switch 1 is opened, the effective resonant selector circuit 3--5 will also have a transmission band of 150 kc. Of course, the subsequent selector circuits in the FM receiver will each have a wide band transmission characteristic, and, furthermore, will each have means for providing Va resonant frequency equal to 42.4 mc. Those skilled in the art will appreciate that if the receiver I is of the superheterodyne type then the local oscillator will have its frequency adjusted simultaneously with closure of switch 6. In this way, there will be provided the proper local oscillation frequency to provide with signals of the mean frequency of 42.4 mc. the operating intermediate frequency (I. F.) of receiver I.

The AM adapter System will comprise a grounded antenna circuit 20 adapted to collect the usual AM broadcast signals. These collected broadcast signals are fed to any well known form of converter stage 2| which is provided with. a tunable signal selector circuit 22 and a local oscillator tank circuit 23. This is well known prac- 4 tice in superheterodyne receivers, and need not be gone into in further detail. It is suflicient to point out that the selector circuit 22 will be tunable over the 550 to 1700 kc. broadcast range. The oscillator circuit 23 will simultaneously be tunable over a range of local oscillation frequencies such that at any setting of the signal selector Imechanism there will be produced the same I. F. value of, for example, 455 kc. The I. F. energy may be fed to a further converter circuit, or frequency changer circuit, designated by numeral 24.

The network 24 is fed with oscillations produced by an oscillator 25 operating at a fixed frequency of 42 mc. In the output circuit of the network 24 there is provided a resonant circuit 26 which is xedly tuned to either the sum or the difference of the frequency of the I. F. energy fed to converter 24 and the frequency of oscillations produced by oscillator 25. It will be understood, of course, that the oscillations provided by oscillator 25 are of constant amplitude and constant frequency. In other words, circuit 26 may be tuned either to 42.45 mc. or 41.55 mc. By way of illustration, let it be assumed that the circuit 26 is tuned to a frequency of 42.45 mc. This means that there will be developed across circuit 26 amplitude modulated carrier energy whose carrier frequency has a value of 42.45 mc. This amplitude modulated carrier energy, which has been frequency changed from the originally collected AM signal energy, may be applied through the series-arranged coupling condenser 21 and the switch I to the input circuit 2 of the FM receiver, provided the latter is properly adjusted. If the switch i6 is closed, and concurrently the switch 9 opened, while switch 1 is opened and switch 6 is concurrently closed, there will be applied to the effective resonant input circuit 3 5 AM signal energy whose carrier frequency is 42.45 mc.

There is simultaneously fed to the resonant input circuit 3 5 unmodulated oscillations of constant amplitude and constant frequency value of 42.4 mc. The unmodulated oscillator is designated by numeral 28, and is shown feeding its oscillatory output through coupling condenser 29 to the switch II). The result of feeding the AM signal energy through the condenser 21 and the unmodulated oscillations from the oscillator 28 through condenser 29, is to provide in the resonant input circuit 3--5 resultant FM signal energy having a mean frequency of 42.4 mc. However, the frequency deviations of the latter correspond to the amplitude modulation which existed on the original carrier collected at the signal collector device 26. In other words, the unmodulated oscillations produced by oscillator 28 have been modulated in frequency in accordance with the modulation of the amplitude modulated signal energy developed across circuit 26.

It will be obvious, therefore, that the resultant frequency modulated energy will be treated in the receiver I in the same manner as if the receiver were operating to receive the FM signal energy in its normal 40-50 mc. band. It will, also, be appreciated that to switch over from FM reception to AM reception the following adjustments need to be made. The switches 9 and I0 are concurrently actuated so as to open switch 9 and close switch I0. As previously explained, opening switch 9 disconnects input coil 9 from dipole 8, while closing switch I 0 electrically connects the circuit 26 to circuit 2 through condenser 21. At the same time switches 1 and 6 are concurrently actuated so as to open switch 1 and close switch Opening switch 'l1 disconnects condenser 4 from across coilL 3., while closing switch 6 electrically connects condenser 5 across coil 3T. The effective-resonant circuit 3, 5l is tuned to 4224 mc., which is the frequency of the urnnodulatedt oscill'ator 23;

In' Fig. 2 there' is shown a vectorial analysis of the relations between the` AM carrier energy and' the unmo'dul'ated carrier energy. The vector Ei is to be considered as the AM carrier whose frequency is 42145- mc. The vector E0l represents the` unmodu'lated carrier whose frequency is 42.4 mc. As represented here the vector Eiis considered as being added to the vector Et. The unmodulated carrier vector En is taken asthe reference frequency so that vectorE1- is consideredl as rotatingA with respect to vector lilo` andL in a clockwise direction.

Reference is made to Fig. 3i to illustrate the relations between the voltages E and Eli from another' viewpoint. The solid line curve represents the band pass characteristic of circuit 3*-5 when switch d is closed and switch l is' opened. It will'4 be seen that the carrier frequency of E1 is located within the aforementioned band', and is located closely adjacent to Et. The voltage E1 is much the weakerof the two. While it is desirable to have the frequency of E1 as close to' the En frequency as possible, it is to be understood that E1 may be located anywhere-within the pass band. A more detailed explanation of Fig. 2 is now given.

The resultant vector of the combination of vector E1 and vector Eb is Er which follows the locus circlezC. It can be seen that theamplitude and the phase of this resultant are modulated as E1 rotates with respect to Eo. That is, vector Er varies in angular position (phase modulationl and amplitude (amplitude modulation); The normal' adjustment of the relative amplitudes of Eo; and Ei is such that Eo is large compared to E1. This makes' the resulting amplitude and phase modulations linear. If the vector E11 were increased or decreased in amplitude, such as would be the case when amplitude modulation is applied, it is apparent that the degree of amplitude and phase modulation occurring on the resulta-nt vector Er would be correspondingly increased or decreased. Hence, if the phase modulation component of the modulation of the resultant vector were detected, the amplitude of the detected cornponent would be amplitude modulated. This phase modulation component consists of a beat note which is equal to the difference between the frequencies of the two voltages Eu and E1. In the above-mentioned egample, it would be 42.45 mc. minus 42.4 mc. or 50` kc. This 50 kc. beat note is amplitude modulated in accordance with the amplitude modulations on the converted amplitude modulation wave at 42.45 mc.

When the phase and amplitude modulated resultant is f ed to the frequency modulation receiver l of Fig. l, the amplitude modulation component of the resultant is removed by the usual amplitude limiter in the FM receiver. The remaining modulation is a phase modulation which may be considered as a frequency modulation due to the similarities between the two types of modulation. The frequency modulation discriminator con-- Verts the phase modulation into amplitudermodulation, which is detected in the rectiiiers normally following the discriminator. Thus, in the case of the example, the 5i) kc. beat note would be detected by the FM rectiiers. Since this beat note is amplitude modulated in accordance with the desired incoming amplitude modulated signal,

'the signal energy of 42.45 mc.

6. its' presence iin'J the detector' circuit'. causes.: the amplitude modulation on the beat: note to Abe detected-. 'Iliatis, the desired signaling. wave spresent as amplitude modulation on' the incoming' amplitude modulated: signah. will.l ap:-

pear as detected output from the detector circuit.

Inl other words, theFMreceiver. detector performs a dual detectionz First, it detects the M kc. beat note; andi then it detects the amplitud'e.K modulation: present. oni the Birke. bicat note. This dual detection action l? have foundto be true by experinient.-

linl 4 I have` shownA a modicatiom The adapter' converter 2132 has its output circuit 26 through: condenser lil into a combining circuit E oscillator 23 feeds its unmoduiated" oscillations into the combining circuiti 30. There i"sfed to converter 24 ocillationsf from an oscillator operating at 41.45v mc. This oscillator isf designated" by numeral 3l. A radio frequency amplie'r 377 couplesthe antenna circuit 2B tothe input circuit of converter 2li. The output circuit of combining' circuit 3i)` is designatedV by numeral 3d, and" there is developed across the resonant output circuit 33`f the resultant of E11 and En. Assuming that an AM signal of 1,060 kc. has been received by collector circuit 2U, the converter 24 willi function toprovid'e across resonant circuit 26 In the combining circuit lill; which'may be of any well k-noWn form, Er andy Eo arev combined to provide across resonant circuit 313 the frequency modulated energy whose mean frequency is 42.4 mc.

Inreceiving amplitude modulated signals with an FM receiver according to this method, the cle-emphasis circuits, which areY now used in the conventional FM broadcast receivers, would have to be switched out. This is required since the amplitude modulated signals normally donot use pre-emphasis at the transmitter. In a receiver asA4 shown in- Eig. l, such a switch would be ganged with switches 6-'l-9-| ll. In case pre-emphasis nis used at the' A-M transmitter, such a switch would be unnecessary. Also, in the type of FM receiver where de-emphasis is not used, such a switch would be unnecessary. By pre-emphasis is meant the emphasizing of the higher audio frequencies, and by de-emphasis is meant the reverse. These terms are well known. It will be understood that the frequency converting systems shown in Figs. 1 and 4 are not a, necessity. If the frequency range of the FM receiver is capable of tuning in the amplitude modulated signals directly, all that is necessary is the combining oscillation of a frequency slightly different than the desired AM signal and having an amplitude greater than the desired AM signal.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention.

What I claim is:

1. In combination with a frequency modulation receiver having a selective input circuit tuned to a predetermined frequency, an adapter system which comprises means for translating received amplitude modulated carrier energy into corresponding amplitude modulated carrier energy whose carrie-r Afrequency falls within the band width of said selective input circuit, oscillator means for producing unmodulated oscillations of a constant frequency equal to the frequency of said selective input circuit, and means for combining said translated amplitude modulated carrier energy and said unmodulated oscillations to produce frequency modulated carrier energy for application to said selective input circuit.

2. In combination with a frequency modulation receiver having a selective input circuit adapted to be tuned to a predetermined frequency of a desired frequency range, an amplitude modulation adapter which comprises means for translating received amplitude modulated carrierv energy into corresponding amplitude modulated carrier energy Whose carrier frequency falls Within the band Width of said selective input circuit but is different from said predetermined frequency, means for producing unmodulated oscillations of a constant frequency equal to the frequency of said selective input circuit, and means for combining said translated amplitude modulated carrier energy and said unmodulated oscillations thereby to produce for application to said selective input circuit resultant frequency modulated waves of a mean frequency equal to said unmodulated oscillation frequency.

'3. A method of translating amplitude modulated carrier energy into frequency modulated carrier energy which includes collecting desired amplitude modulated carrier energy, converting the latter to amplitude modulated carrier energy of a higher carrier frequency, producing unmodulated oscillations of a constant frequency differing by a relatively small frequency value from the frequency of the converted amplitude modulated carrier frequency, combining said converted amplitude modulated carrier energy with said unmodulated oscillations thereby to provide correspondingly frequency modulated oscillations Whose mean frequency is the frequency of said unmodulated oscillations, and detecting the frequency modulated oscillations to provide the original modulation.

4. In combination with a frequency modulation receiver having a selective input circuit tuned to a predetermined frequency, means for translating received amplitude modulated carrier energy into corresponding amplitude modulated carrier energy whose carrier frequency falls within the band width of said selective input circuit, means for producing unmodulated oscillations of a constant frequency equal to the frequency of said selective input circuit, and means for combining said translated amplitude modulated carrier energy and said unmodulated oscillations to produce frequency modulated carrier energy for application to said selective input circuit.

5. In combination With an angle modulated AWave receiver having a selective input circuit tuned to a predetermined frequency, an adapter which comprises means for providing amplitude modulated carrier energy whose carrier frequency falls within the pass band of said selective input circuit, means for producing unmodulated oscillations of a constant frequency equal to the frequency of said selective input circuit, and means for combining said amplitude modulated carrier energy and said unmodulated oscillations to angle modulate said oscillations in accordance with the modulation of said amplitude modulated carrier energy.

MURRAY G. CROSBY.

REFERENCES CTED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,941,068 Armstrong Dec. 26, 1933 2,151,810 Siemens Mar. 28, 1939 2,215,284 Armstrong Sept. 17, 1940 2,287,044 Kroger June 23, 1942 2,334,189 Goldstine Nov, 16, 1943 2,378,581 Roberts June 19, 1945 

